The solar system consists of the sun, the planets, their satellites (moons), and other smaller bodies such as asteroids, comets, and meteoroids.
The asteroids are small objects that follow separate orbits between Mars and Jupiter.
Planets revolve around the sun and rotate on their axes.
All revolutions and rotations, except for Venus and a few minor satellites, are in the same direction. Uranus rotates about an axis only 8° from the plane of its orbit.
All the orbits except those of Pluto and the comets lie nearly in the same plane.
The inner planets are Mercury, Venus, Earth, and Mars.
The inner planets are relatively small and rocky, have similar densities, have low escape speeds, and rotate fairly slowly on their axes.
Of the inner planet satellites, the earth's moon is the only satellite of any size.
The outer planets are Jupiter, Saturn, Uranus, Neptune, and Pluto.
All the outer planets except Pluto are large, not very dense, rotate fairly rapidly, have high escape speeds, and are composed mostly of liquefied gases (chiefly hydrogen and helium).
The outer planets have a total of at least 130 satellites.
Pluto is a small planet composed of rock and frozen gases.
16-2. Comets
Comets are leftover matter from the early history of the solar system and consist mainly of ice and dust.
Most comets move in long, narrow elliptical orbits about the sun.
Comets are visible only when close to the sun partly because of sunlight scattered by cometary material but mainly through the excitation of vaporized gases by solar ultraviolet radiation.
A comet's tail always points away from the sun because of pressure from the sun's radiation on the comet's dust particles and from the solar wind (the stream of ions flowing outward from the sun) sweeping the comet's gases with it.
16-3. Meteors
Meteoroids are small fragments of matter, formed as a result of asteroid collisions and from comet debris, that the earth meets as it travels through space.
Meteors, or "shooting stars," are the glows of meteoroids as they pass through the earth's atmosphere.
Meteorites are meteoroids that have fallen to earth.
Micrometeorites are fine, dustlike meteorites that float through the earth's surface without burning up.
Meteor showers occur at specific times of the year when the earth moves through a swarm of meteoroids that all follow the same orbit about the sun.
Most meteorites fall into two classes:
Stony meteorites, which consist of silicate minerals.
Iron meteorites, which consist largely of iron and some nickel.
The sizes and rate of arrival of meteorites bombarding earth and earth's neighbors in space fell to present rates about 3 billion years ago.
16-4. Mercury
Mercury is the planet closest to the sun.
Mercury takes 59 earth days to complete one rotation.
Mercury has almost no atmosphere. Temperatures fall to -180°C on the dark side and reach 425°C on the sunlit side.
The surface of Mercury is pocked by meteoroid craters.
16-5. Venus
Venus is the brightest object in the sky apart from the sun and moon.
Venus rotates clockwise (looking down from above its north pole), unlike the earth and the other planets which rotate counterclockwise.
A day on Venus is 243 earth days long because of the planet's slow rotation.
Venus resembles the earth in size and shape; however, the atmospheric pressure is about 90 times that of earth, and the surface temperature averages over 400°C.Under such extreme conditions, life on Venus seems impossible.
16-6. Mars
Mars is second only to Venus in brightness and is reddish orange in color.
Mars's satellites, Phobos and Deimos, discovered by Asaph Hall in 1877, are very small.
Mars's diameter is slightly over half that of the earth, and its mass is 11 percent of the earth's mass.
Mars probably has about the same composition as the earth.
Mars's period of rotation is a little over 24 h; its period of revolution requires nearly two earth years.
The Martian day and night are about the same length as earth's; however, the seasons are 6 months long.
16-7. Is There Life on Mars?
Conditions on Mars are extremely harsh.
Mars receives much less solar energy than does earth.
Mars's atmosphere is extremely thin and consists largely of carbon dioxide.
Mars's atmosphere retains very little heat after nightfall and is unable to screen out harmful solar ultraviolet radiation.
Liquid water is scarce on Mars, although it might have once been abundant.
Despite the harsh conditions, life of some kind is conceivable on Mars, although no evidence of present or past life has been detected in photographs or by the Viking spacecraft that landed on Mars in 1976.
Conditions suitable for life possibly once existed on Mars and may do so today.
16-8. Asteroids
Asteroids are small, rocky bits of matter from the early solar system that never became part of larger bodies.
Most asteroids orbit the sun in a belt between Mars and Jupiter.
Ceres, the largest asteroid and the first to be identified, was discovered by the Italian astronomer Giuseppe Piazzi.
Several thousand asteroids have been tracked and named.
Asteroids have collided with earth in the past and will do so again.
16-9. Jupiter
Jupiter is the largest planet.
The Great Red Spot is a huge atmospheric disturbance. It is probably the same spot described by the French astronomer Cassini who used it to determine Jupiter's period of rotation.
Jupiter's period of rotation is a little less than 10 h.
Jupiter has 61 known satellites; the largest is as big as Mercury, the smallest about 2 km in diameter.
All of Jupiter’s moons, except Callisto, show signs of geological activity.
The four outermost satellites revolve "backward" around Jupiter.
Jupiter has a faint ring.
Jupiter's volume is about 1300 times that of the earth, but its mass is only 318 times as great.
Jupiter has a magnetic field nearly 10 times stronger than the earth's.
Jupiter radiates over twice as much energy as it receives from the sun.
16-10. Saturn
Saturn is much like Jupiter in many respects, though smaller and less massive.
There are at least 18 satellites of Saturn, the largest being Titan, which is the only satellite in the solar system with an atmosphere.
A number of rings surround Saturn at its equator.
The rings consist of numerous small bodies each of which revolves around Saturn as a miniature satellite.
In 1980 and 1981 Voyager spacecraft reached Saturn and reported that the bodies that compose the rings are chunks of rock and ice.
16-11. Uranus, Neptune, Pluto
Uranus was discovered in 1781, Neptune in 1846, and Pluto in 1930.
Uranus rotates about an axis only 8° from the plane of its orbit.
The Voyager 2 spacecraft reported the presence of rings around Uranus and Neptune.
Neptune's atmosphere is more stormy than that of Uranus.
Pluto seems to be about two-thirds the size of the moon and to consist of rock, ice, and frozen methane.
Pluto's satellite, Charon, has a diameter half that of Pluto and might have an ocean of liquid water under a covering of ice.
16-12. Phases
The moon is earth's satellite.
The distance from the earth to the moon averages 384,000 km.
The moon has a diameter of 3476 km.
The moon circles the earth every 271/3 days.
Since the moon's rotation keeps pace exactly with its revolution of the earth, the same hemisphere of the moon always faces the earth.
The light from the moon is reflected sunlight.
Relative to the stars, the moon's orbital period is 271/3 days; relative to the sun, it is 291/2 days.
During each 291/2-day period the moon goes through its cycle of phases.
The phases represent the amount of the moon's illuminated surface visible to us in different parts of its orbit.
A full moon is on the opposite side of the earth from the sun, so the side facing earth is fully illuminated.
A new moon is moving approximately between us and the sun, so the side facing earth is in shadow.
The new moon is not completely dark because of earthshine—sunlight reflected from the earth’s surface that reaches the moon.
16-13. Eclipses
Eclipses occur when the moon passes exactly between the earth and the sun and obscures the sun (solar eclipse) or when the moon passes exactly behind the earth so that earth's shadow obscures the moon (lunar eclipse).
The circular shape of the earth's shadow during a lunar eclipse is evidence for earth's spherical shape.
Partial eclipses of the sun take place when the moon is not exactly aligned with the sun, so that only part of the sun's disk is obscured. During a total eclipse, the moon completely blocks out light from the sun.
An annular eclipse of the sun occurs when the moon is farthest from the earth and its apparent diameter is less than that of the sun. The result is a ring of sunlight appearing around the rim of the moon.
16-14. The Lunar Surface
The moon has no atmosphere and no surface water.
The two main kinds of lunar landscape include:
The dark, relatively smooth maria (the singular is mare)
The lighter, ruggedly mountainous highlands
The maria consist of lava flows that have been broken up by meteoroid impacts.
The lunar highlands are scarred by numerous craters.
Certain craters have conspicuous streaks of light-colored matter radiating outward called rays.
The rays consist of lunar material sprayed outward after meteoroid impacts.
Lunar rilles of the highlands are narrow channels that look like dried-up riverbeds and were probably created by the collapse of subsurface channels through which lava once flowed.
Seismographic data reveal that the moon has a rigid crust, a thick solid mantle, and a small, dense core.
16-15. Evolution of the Lunar Landscape
The analysis of lunar rock and soil samples has led to a number of conclusions about the history of the moon.
Some lunar rocks solidified soon after the solar system came into being.
The moon's landscape was shaped by meteoroid bombardment and volcanic activity.
The youngest rocks found on the moon are 3 billion years old, indicating that all igneous activity stopped then.
Meteoroids continue to crater the lunar landscape.
16-16. Origin of the Moon
Until recently theories of the moon fell into three categories:
The moon split away from the earth.
The moon was formed elsewhere and later captured by the earth's gravitational field.
The moon and the earth came into being together as a double-planet system.
All three theories have serious weaknesses.
A fourth proposal, the collision hypothesis, is widely accepted.
The collision hypothesis suggests that another planet, a little larger than Mars and with a slightly different composition from that of the earth, crashed into the earth.
The mantle of the other planet and some of the earth's mantle were thrown off into orbit to form the moon.
The other planet's iron core was added to the earth's core.
